Hitherto, as an engine of an automatic two-wheeled vehicle, there is known an engine including a variable valve mechanism changing operation characteristics (a valve opening/closing timing or a valve lift amount) of an intake valve and an exhaust valve in response to an engine rotation speed (for example, see JP 2012-225277 A). The variable valve mechanism described in JP 2012-225277 A is applied to a SOHC type valve train. Specifically, in JP 2012-225277 A, one cam shaft is provided with two kinds of cams (a low-speed cam and a high-speed cam) having different operation characteristics. Further, a rocker arm which drives an intake/exhaust valve is slidable in the axial direction of the cam shaft. When the rocker arm slides in response to an engine rotation speed, the low-speed cam and the high-speed cam can be switched. Thus, it is possible to obtain desired operation characteristics of the valve mechanism.